OBJECTIVE: Although maternal stress, anxiety, and depression have been linked to negative birth outcomes, few studies have investigated preventive interventions targeting maternal mental health as a means of reducing such problems. This randomized controlled study examines whether Family Foundations (FF)-a transition to parenthood program for couples focused on promoting coparenting quality, with previously documented impact on maternal stress, depression, and anxiety-can buffer the negative effects of maternal mental health problems. METHODS: To assess the effects of FF, we used a randomized block design with a sample of 259 expectant mothers assigned to FF or a control condition and analyzed using propensity score models. We examine two-way interactions of condition (intervention vs. control) with maternal mental health problems (financial stress, depression, and anxiety) on birth outcomes (birth weight, days in hospital for mothers and infants). For birth weight, we assess whether intervention effects depend on length of gestation by including a third interaction term. RESULTS: FF buffered (p < 0.05) the negative impact of maternal mental health problems on birth weight and both mother and infant length of post-partum hospital stay. For birth weight, assignment to FF was associated with higher birth weight for infants born before term. CONCLUSIONS: These results demonstrate that a psycho-educational program for couples focused on enhancing mutual coparental support, with preventive effects on maternal mental health, can reduce incidence of birth problems among women at elevated risk. Such improvements in birth outcomes could translate into substantial reductions in public and personal healthcare costs. Future work should assess mediating mechanisms of intervention impact and cost-benefit ratio of the intervention. CLINICAL TRIALS REGISTRATION: The Family Foundations follow-up intervention study is currently registered with www.clinicaltrials.gov . The study identifier is NCT01907412.